/* Hash Tables Implementation.
 *
 * This file implements in memory hash tables with insert/del/replace/find/
 * get-random-element operations. Hash tables will auto resize if needed
 * tables of power of two in size are used, collisions are handled by
 * chaining. See the source code for more information... :)
 *
 * Copyright (c) 2006-2012, Salvatore Sanfilippo <antirez at gmail dot com>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include "../lib/fmacros.h"

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include <limits.h>
#include <sys/time.h>
#include <ctype.h>
#include <assert.h>

#include "../lib/dict.h"
#include "../lib/zmalloc.h"

/* Using dictEnableResize() / dictDisableResize() we make possible to
 * enable/disable resizing of the hash table as needed. This is very important
 * for Redis, as we use copy-on-write and don't want to move too much memory
 * around when there is a child performing saving operations.
 *
 * Note that even when dict_can_resize is set to 0, not all resizes are
 * prevented: a hash table is still allowed to grow if the ratio between
 * the number of elements and the buckets > dict_force_resize_ratio. */
static int dict_can_resize = 1;
static unsigned int dict_force_resize_ratio = 5;

/* -------------------------- private prototypes ---------------------------- */

static int _dictExpandIfNeeded ( dict *ht );
static unsigned long _dictNextPower ( unsigned long size );
static int _dictKeyIndex ( dict *ht, const void *key );
static int _dictInit ( dict *ht, dictType *type, void *privDataPtr );

/* -------------------------- hash functions -------------------------------- */

/* Thomas Wang's 32 bit Mix Function */
unsigned int dictIntHashFunction ( unsigned int key )
{
    key += ~ ( key << 15 );
    key ^= ( key >> 10 );
    key += ( key << 3 );
    key ^= ( key >> 6 );
    key += ~ ( key << 11 );
    key ^= ( key >> 16 );
    return key;
}

static uint32_t dict_hash_function_seed = 5381;

void dictSetHashFunctionSeed ( uint32_t seed )
{
    dict_hash_function_seed = seed;
}

uint32_t dictGetHashFunctionSeed ( void )
{
    return dict_hash_function_seed;
}

/* MurmurHash2, by Austin Appleby
 * Note - This code makes a few assumptions about how your machine behaves -
 * 1. We can read a 4-byte value from any address without crashing
 * 2. sizeof(int) == 4
 *
 * And it has a few limitations -
 *
 * 1. It will not work incrementally.
 * 2. It will not produce the same results on little-endian and big-endian
 *    machines.
 */
unsigned int dictGenHashFunction ( const void *key, int len )
{
    /* 'm' and 'r' are mixing constants generated offline.
     They're not really 'magic', they just happen to work well.  */
    uint32_t seed = dict_hash_function_seed;
    const uint32_t m = 0x5bd1e995;
    const int r = 24;

    /* Initialize the hash to a 'random' value */
    uint32_t h = seed ^ len;

    /* Mix 4 bytes at a time into the hash */
    const unsigned char *data = ( const unsigned char * ) key;

    while ( len >= 4 )
    {
        uint32_t k = * ( uint32_t* ) data;

        k *= m;
        k ^= k >> r;
        k *= m;

        h *= m;
        h ^= k;

        data += 4;
        len -= 4;
    }

    /* Handle the last few bytes of the input array  */
    switch ( len )
    {
        case 3: h ^= data[2] << 16;
        case 2: h ^= data[1] << 8;
        case 1: h ^= data[0];
            h *= m;
    };

    /* Do a few final mixes of the hash to ensure the last few
     * bytes are well-incorporated. */
    h ^= h >> 13;
    h *= m;
    h ^= h >> 15;

    return ( unsigned int ) h;
}

/* And a case insensitive hash function (based on djb hash) */
unsigned int dictGenCaseHashFunction ( const unsigned char *buf, int len )
{
    unsigned int hash = ( unsigned int ) dict_hash_function_seed;

    while ( len -- )
        hash = ( ( hash << 5 ) + hash ) + ( tolower (*buf ++) ); /* hash * 33 + c */
    return hash;
}

/* ----------------------------- API implementation ------------------------- */

/* Reset a hash table already initialized with ht_init().
 * NOTE: This function should only be called by ht_destroy(). */
static void _dictReset ( dictht *ht )
{
    ht->table = NULL;
    ht->size = 0;
    ht->sizemask = 0;
    ht->used = 0;
}

/* Create a new hash table */
dict *dictCreate ( dictType *type,
                   void *privDataPtr )
{
    dict *d = zmalloc (sizeof (*d ));

    _dictInit (d, type, privDataPtr);
    return d;
}

/* Initialize the hash table */
int _dictInit ( dict *d, dictType *type,
                void *privDataPtr )
{
    _dictReset (&d->ht[0]);
    _dictReset (&d->ht[1]);
    d->type = type;
    d->privdata = privDataPtr;
    d->rehashidx = - 1;
    d->iterators = 0;
    return DICT_OK;
}

/* Resize the table to the minimal size that contains all the elements,
 * but with the invariant of a USED/BUCKETS ratio near to <= 1 */
int dictResize ( dict *d )
{
    int minimal;

    if ( ! dict_can_resize || dictIsRehashing (d) ) return DICT_ERR;
    minimal = d->ht[0].used;
    if ( minimal < DICT_HT_INITIAL_SIZE )
        minimal = DICT_HT_INITIAL_SIZE;
    return dictExpand (d, minimal);
}

/* Expand or create the hash table */
int dictExpand ( dict *d, unsigned long size )
{
    dictht n; /* the new hash table */
    unsigned long realsize = _dictNextPower (size);

    /* the size is invalid if it is smaller than the number of
     * elements already inside the hash table */
    if ( dictIsRehashing (d) || d->ht[0].used > size )
        return DICT_ERR;

    /* Rehashing to the same table size is not useful. */
    if ( realsize == d->ht[0].size ) return DICT_ERR;

    /* Allocate the new hash table and initialize all pointers to NULL */
    n.size = realsize;
    n.sizemask = realsize - 1;
    n.table = zcalloc (realsize * sizeof (dictEntry* ));
    n.used = 0;

    /* Is this the first initialization? If so it's not really a rehashing
     * we just set the first hash table so that it can accept keys. */
    if ( d->ht[0].table == NULL )
    {
        d->ht[0] = n;
        return DICT_OK;
    }

    /* Prepare a second hash table for incremental rehashing */
    d->ht[1] = n;
    d->rehashidx = 0;
    return DICT_OK;
}

/* Performs N steps of incremental rehashing. Returns 1 if there are still
 * keys to move from the old to the new hash table, otherwise 0 is returned.
 *
 * Note that a rehashing step consists in moving a bucket (that may have more
 * than one key as we use chaining) from the old to the new hash table, however
 * since part of the hash table may be composed of empty spaces, it is not
 * guaranteed that this function will rehash even a single bucket, since it
 * will visit at max N*10 empty buckets in total, otherwise the amount of
 * work it does would be unbound and the function may block for a long time. */
int dictRehash ( dict *d, int n )
{
    int empty_visits = n * 10; /* Max number of empty buckets to visit. */
    if ( ! dictIsRehashing (d) ) return 0;

    while ( n -- && d->ht[0].used != 0 )
    {
        dictEntry *de, *nextde;

        /* Note that rehashidx can't overflow as we are sure there are more
         * elements because ht[0].used != 0 */
        assert (d->ht[0].size > ( unsigned long ) d->rehashidx);
        while ( d->ht[0].table[d->rehashidx] == NULL )
        {
            d->rehashidx ++;
            if ( -- empty_visits == 0 ) return 1;
        }
        de = d->ht[0].table[d->rehashidx];
        /* Move all the keys in this bucket from the old to the new hash HT */
        while ( de )
        {
            unsigned int h;

            nextde = de->next;
            /* Get the index in the new hash table */
            h = dictHashKey (d, de->key) & d->ht[1].sizemask;
            de->next = d->ht[1].table[h];
            d->ht[1].table[h] = de;
            d->ht[0].used --;
            d->ht[1].used ++;
            de = nextde;
        }
        d->ht[0].table[d->rehashidx] = NULL;
        d->rehashidx ++;
    }

    /* Check if we already rehashed the whole table... */
    if ( d->ht[0].used == 0 )
    {
        zfree (d->ht[0].table);
        d->ht[0] = d->ht[1];
        _dictReset (&d->ht[1]);
        d->rehashidx = - 1;
        return 0;
    }

    /* More to rehash... */
    return 1;
}

long long timeInMilliseconds ( void )
{
    struct timeval tv;

    gettimeofday (&tv, NULL);
    return (( ( long long ) tv.tv_sec )*1000 )+( tv.tv_usec / 1000 );
}

/* Rehash for an amount of time between ms milliseconds and ms+1 milliseconds */
int dictRehashMilliseconds ( dict *d, int ms )
{
    long long start = timeInMilliseconds ();
    int rehashes = 0;

    while ( dictRehash (d, 100) )
    {
        rehashes += 100;
        if ( timeInMilliseconds () - start > ms ) break;
    }
    return rehashes;
}

/* This function performs just a step of rehashing, and only if there are
 * no safe iterators bound to our hash table. When we have iterators in the
 * middle of a rehashing we can't mess with the two hash tables otherwise
 * some element can be missed or duplicated.
 *
 * This function is called by common lookup or update operations in the
 * dictionary so that the hash table automatically migrates from H1 to H2
 * while it is actively used. */
static void _dictRehashStep ( dict *d )
{
    if ( d->iterators == 0 ) dictRehash (d, 1);
}

/* Add an element to the target hash table */
int dictAdd ( dict *d, void *key, void *val )
{
    dictEntry *entry = dictAddRaw (d, key);

    if ( ! entry ) return DICT_ERR;
    dictSetVal (d, entry, val);
    return DICT_OK;
}

/* Low level add. This function adds the entry but instead of setting
 * a value returns the dictEntry structure to the user, that will make
 * sure to fill the value field as he wishes.
 *
 * This function is also directly exposed to the user API to be called
 * mainly in order to store non-pointers inside the hash value, example:
 *
 * entry = dictAddRaw(dict,mykey);
 * if (entry != NULL) dictSetSignedIntegerVal(entry,1000);
 *
 * Return values:
 *
 * If key already exists NULL is returned.
 * If key was added, the hash entry is returned to be manipulated by the caller.
 */
dictEntry *dictAddRaw ( dict *d, void *key )
{
    int index;
    dictEntry *entry;
    dictht *ht;

    if ( dictIsRehashing (d) ) _dictRehashStep (d);

    /* Get the index of the new element, or -1 if
     * the element already exists. */
    if ( ( index = _dictKeyIndex (d, key) ) == - 1 )
        return NULL;

    /* Allocate the memory and store the new entry.
     * Insert the element in top, with the assumption that in a database
     * system it is more likely that recently added entries are accessed
     * more frequently. */
    ht = dictIsRehashing (d) ? &d->ht[1] : &d->ht[0];
    entry = zmalloc (sizeof (*entry ));
    entry->next = ht->table[index];
    ht->table[index] = entry;
    ht->used ++;

    /* Set the hash entry fields. */
    dictSetKey (d, entry, key);
    return entry;
}

/* Add an element, discarding the old if the key already exists.
 * Return 1 if the key was added from scratch, 0 if there was already an
 * element with such key and dictReplace() just performed a value update
 * operation. */
int dictReplace ( dict *d, void *key, void *val )
{
    dictEntry *entry, auxentry;

    /* Try to add the element. If the key
     * does not exists dictAdd will suceed. */
    if ( dictAdd (d, key, val) == DICT_OK )
        return 1;
    /* It already exists, get the entry */
    entry = dictFind (d, key);
    /* Set the new value and free the old one. Note that it is important
     * to do that in this order, as the value may just be exactly the same
     * as the previous one. In this context, think to reference counting,
     * you want to increment (set), and then decrement (free), and not the
     * reverse. */
    auxentry = * entry;
    dictSetVal (d, entry, val);
    dictFreeVal (d, &auxentry);
    return 0;
}

/* dictReplaceRaw() is simply a version of dictAddRaw() that always
 * returns the hash entry of the specified key, even if the key already
 * exists and can't be added (in that case the entry of the already
 * existing key is returned.)
 *
 * See dictAddRaw() for more information. */
dictEntry *dictReplaceRaw ( dict *d, void *key )
{
    dictEntry *entry = dictFind (d, key);

    return entry ? entry : dictAddRaw (d, key);
}

/* Search and remove an element */
static int dictGenericDelete ( dict *d, const void *key, int nofree )
{
    unsigned int h, idx;
    dictEntry *he, *prevHe;
    int table;

    if ( d->ht[0].size == 0 ) return DICT_ERR; /* d->ht[0].table is NULL */
    if ( dictIsRehashing (d) ) _dictRehashStep (d);
    h = dictHashKey (d, key);

    for ( table = 0; table <= 1; table ++ )
    {
        idx = h & d->ht[table].sizemask;
        he = d->ht[table].table[idx];
        prevHe = NULL;
        while ( he )
        {
            if ( key == he->key || dictCompareKeys (d, key, he->key) )
            {
                /* Unlink the element from the list */
                if ( prevHe )
                    prevHe->next = he->next;
                else
                    d->ht[table].table[idx] = he->next;
                if ( ! nofree )
                {
                    dictFreeKey (d, he);
                    dictFreeVal (d, he);
                }
                zfree (he);
                d->ht[table].used --;
                return DICT_OK;
            }
            prevHe = he;
            he = he->next;
        }
        if ( ! dictIsRehashing (d) ) break;
    }
    return DICT_ERR; /* not found */
}

int dictDelete ( dict *ht, const void *key )
{
    return dictGenericDelete (ht, key, 0);
}

int dictDeleteNoFree ( dict *ht, const void *key )
{
    return dictGenericDelete (ht, key, 1);
}

/* Destroy an entire dictionary */
int _dictClear ( dict *d, dictht *ht, void(callback ) ( void * ) )
{
    unsigned long i;

    /* Free all the elements */
    for ( i = 0; i < ht->size && ht->used > 0; i ++ )
    {
        dictEntry *he, *nextHe;

        if ( callback && ( i & 65535 ) == 0 ) callback (d->privdata);

        if ( ( he = ht->table[i] ) == NULL ) continue;
        while ( he )
        {
            nextHe = he->next;
            dictFreeKey (d, he);
            dictFreeVal (d, he);
            zfree (he);
            ht->used --;
            he = nextHe;
        }
    }
    /* Free the table and the allocated cache structure */
    zfree (ht->table);
    /* Re-initialize the table */
    _dictReset (ht);
    return DICT_OK; /* never fails */
}

/* Clear & Release the hash table */
void dictRelease ( dict *d )
{
    _dictClear (d, &d->ht[0], NULL);
    _dictClear (d, &d->ht[1], NULL);
    zfree (d);
}

dictEntry *dictFind ( dict *d, const void *key )
{
    dictEntry *he;
    unsigned int h, idx, table;

    if ( d->ht[0].used + d->ht[1].used == 0 ) return NULL; /* dict is empty */
    if ( dictIsRehashing (d) ) _dictRehashStep (d);
    h = dictHashKey (d, key);
    for ( table = 0; table <= 1; table ++ )
    {
        idx = h & d->ht[table].sizemask;
        he = d->ht[table].table[idx];
        while ( he )
        {
            if ( key == he->key || dictCompareKeys (d, key, he->key) )
                return he;
            he = he->next;
        }
        if ( ! dictIsRehashing (d) ) return NULL;
    }
    return NULL;
}

void *dictFetchValue ( dict *d, const void *key )
{
    dictEntry *he;

    he = dictFind (d, key);
    return he ? dictGetVal (he) : NULL;
}

/* A fingerprint is a 64 bit number that represents the state of the dictionary
 * at a given time, it's just a few dict properties xored together.
 * When an unsafe iterator is initialized, we get the dict fingerprint, and check
 * the fingerprint again when the iterator is released.
 * If the two fingerprints are different it means that the user of the iterator
 * performed forbidden operations against the dictionary while iterating. */
long long dictFingerprint ( dict *d )
{
    long long integers[6], hash = 0;
    int j;

    integers[0] = ( long ) d->ht[0].table;
    integers[1] = d->ht[0].size;
    integers[2] = d->ht[0].used;
    integers[3] = ( long ) d->ht[1].table;
    integers[4] = d->ht[1].size;
    integers[5] = d->ht[1].used;

    /* We hash N integers by summing every successive integer with the integer
     * hashing of the previous sum. Basically:
     *
     * Result = hash(hash(hash(int1)+int2)+int3) ...
     *
     * This way the same set of integers in a different order will (likely) hash
     * to a different number. */
    for ( j = 0; j < 6; j ++ )
    {
        hash += integers[j];
        /* For the hashing step we use Tomas Wang's 64 bit integer hash. */
        hash = ( ~ hash ) + ( hash << 21 ); // hash = (hash << 21) - hash - 1;
        hash = hash ^ ( hash >> 24 );
        hash = ( hash + ( hash << 3 ) ) + ( hash << 8 ); // hash * 265
        hash = hash ^ ( hash >> 14 );
        hash = ( hash + ( hash << 2 ) ) + ( hash << 4 ); // hash * 21
        hash = hash ^ ( hash >> 28 );
        hash = hash + ( hash << 31 );
    }
    return hash;
}

dictIterator *dictGetIterator ( dict *d )
{
    dictIterator *iter = zmalloc (sizeof (*iter ));

    iter->d = d;
    iter->table = 0;
    iter->index = - 1;
    iter->safe = 0;
    iter->entry = NULL;
    iter->nextEntry = NULL;
    return iter;
}

dictIterator *dictGetSafeIterator ( dict *d )
{
    dictIterator *i = dictGetIterator (d);

    i->safe = 1;
    return i;
}

dictEntry *dictNext ( dictIterator *iter )
{
    while ( 1 )
    {
        if ( iter->entry == NULL )
        {
            dictht *ht = & iter->d->ht[iter->table];
            if ( iter->index == - 1 && iter->table == 0 )
            {
                if ( iter->safe )
                    iter->d->iterators ++;
                else
                    iter->fingerprint = dictFingerprint (iter->d);
            }
            iter->index ++;
            if ( iter->index >= ( long ) ht->size )
            {
                if ( dictIsRehashing (iter->d) && iter->table == 0 )
                {
                    iter->table ++;
                    iter->index = 0;
                    ht = & iter->d->ht[1];
                }
                else
                {
                    break;
                }
            }
            iter->entry = ht->table[iter->index];
        }
        else
        {
            iter->entry = iter->nextEntry;
        }
        if ( iter->entry )
        {
            /* We need to save the 'next' here, the iterator user
             * may delete the entry we are returning. */
            iter->nextEntry = iter->entry->next;
            return iter->entry;
        }
    }
    return NULL;
}

void dictReleaseIterator ( dictIterator *iter )
{
    if ( ! ( iter->index == - 1 && iter->table == 0 ) )
    {
        if ( iter->safe )
            iter->d->iterators --;
        else
            assert (iter->fingerprint == dictFingerprint (iter->d));
    }
    zfree (iter);
}

/* Return a random entry from the hash table. Useful to
 * implement randomized algorithms */
dictEntry *dictGetRandomKey ( dict *d )
{
    dictEntry *he, *orighe;
    unsigned int h;
    int listlen, listele;

    if ( dictSize (d) == 0 ) return NULL;
    if ( dictIsRehashing (d) ) _dictRehashStep (d);
    if ( dictIsRehashing (d) )
    {
        do
        {
            /* We are sure there are no elements in indexes from 0
             * to rehashidx-1 */
            h = d->rehashidx + ( random () % ( d->ht[0].size +
                                               d->ht[1].size -
                                               d->rehashidx ) );
            he = ( h >= d->ht[0].size ) ? d->ht[1].table[h - d->ht[0].size] :
                d->ht[0].table[h];
        }
        while ( he == NULL );
    }
    else
    {
        do
        {
            h = random () & d->ht[0].sizemask;
            he = d->ht[0].table[h];
        }
        while ( he == NULL );
    }

    /* Now we found a non empty bucket, but it is a linked
     * list and we need to get a random element from the list.
     * The only sane way to do so is counting the elements and
     * select a random index. */
    listlen = 0;
    orighe = he;
    while ( he )
    {
        he = he->next;
        listlen ++;
    }
    listele = random () % listlen;
    he = orighe;
    while ( listele -- ) he = he->next;
    return he;
}

/* This function samples the dictionary to return a few keys from random
 * locations.
 *
 * It does not guarantee to return all the keys specified in 'count', nor
 * it does guarantee to return non-duplicated elements, however it will make
 * some effort to do both things.
 *
 * Returned pointers to hash table entries are stored into 'des' that
 * points to an array of dictEntry pointers. The array must have room for
 * at least 'count' elements, that is the argument we pass to the function
 * to tell how many random elements we need.
 *
 * The function returns the number of items stored into 'des', that may
 * be less than 'count' if the hash table has less than 'count' elements
 * inside, or if not enough elements were found in a reasonable amount of
 * steps.
 *
 * Note that this function is not suitable when you need a good distribution
 * of the returned items, but only when you need to "sample" a given number
 * of continuous elements to run some kind of algorithm or to produce
 * statistics. However the function is much faster than dictGetRandomKey()
 * at producing N elements. */
unsigned int dictGetSomeKeys ( dict *d, dictEntry **des, unsigned int count )
{
    unsigned long j; /* internal hash table id, 0 or 1. */
    unsigned long tables; /* 1 or 2 tables? */
    unsigned long stored = 0, maxsizemask;
    unsigned long maxsteps;

    if ( dictSize (d) < count ) count = dictSize (d);
    maxsteps = count * 10;

    /* Try to do a rehashing work proportional to 'count'. */
    for ( j = 0; j < count; j ++ )
    {
        if ( dictIsRehashing (d) )
            _dictRehashStep (d);
        else
            break;
    }

    tables = dictIsRehashing (d) ? 2 : 1;
    maxsizemask = d->ht[0].sizemask;
    if ( tables > 1 && maxsizemask < d->ht[1].sizemask )
        maxsizemask = d->ht[1].sizemask;

    /* Pick a random point inside the larger table. */
    unsigned long i = random () & maxsizemask;
    unsigned long emptylen = 0; /* Continuous empty entries so far. */
    while ( stored < count && maxsteps -- )
    {
        for ( j = 0; j < tables; j ++ )
        {
            /* Invariant of the dict.c rehashing: up to the indexes already
             * visited in ht[0] during the rehashing, there are no populated
             * buckets, so we can skip ht[0] for indexes between 0 and idx-1. */
            if ( tables == 2 && j == 0 && i < ( unsigned long ) d->rehashidx )
            {
                /* Moreover, if we are currently out of range in the second
                 * table, there will be no elements in both tables up to
                 * the current rehashing index, so we jump if possible.
                 * (this happens when going from big to small table). */
                if ( i >= d->ht[1].size ) i = d->rehashidx;
                continue;
            }
            if ( i >= d->ht[j].size ) continue; /* Out of range for this table. */
            dictEntry *he = d->ht[j].table[i];

            /* Count contiguous empty buckets, and jump to other
             * locations if they reach 'count' (with a minimum of 5). */
            if ( he == NULL )
            {
                emptylen ++;
                if ( emptylen >= 5 && emptylen > count )
                {
                    i = random () & maxsizemask;
                    emptylen = 0;
                }
            }
            else
            {
                emptylen = 0;
                while ( he )
                {
                    /* Collect all the elements of the buckets found non
                     * empty while iterating. */
                    *des = he;
                    des ++;
                    he = he->next;
                    stored ++;
                    if ( stored == count ) return stored;
                }
            }
        }
        i = ( i + 1 ) & maxsizemask;
    }
    return stored;
}

/* Function to reverse bits. Algorithm from:
 * http://graphics.stanford.edu/~seander/bithacks.html#ReverseParallel */
static unsigned long rev ( unsigned long v )
{
    unsigned long s = 8 * sizeof (v ); // bit size; must be power of 2
    unsigned long mask = ~ 0;
    while ( ( s >>= 1 ) > 0 )
    {
        mask ^= ( mask << s );
        v = ( ( v >> s ) & mask ) | ( ( v << s ) & ~ mask );
    }
    return v;
}

/* dictScan() is used to iterate over the elements of a dictionary.
 *
 * Iterating works the following way:
 *
 * 1) Initially you call the function using a cursor (v) value of 0.
 * 2) The function performs one step of the iteration, and returns the
 *    new cursor value you must use in the next call.
 * 3) When the returned cursor is 0, the iteration is complete.
 *
 * The function guarantees all elements present in the
 * dictionary get returned between the start and end of the iteration.
 * However it is possible some elements get returned multiple times.
 *
 * For every element returned, the callback argument 'fn' is
 * called with 'privdata' as first argument and the dictionary entry
 * 'de' as second argument.
 *
 * HOW IT WORKS.
 *
 * The iteration algorithm was designed by Pieter Noordhuis.
 * The main idea is to increment a cursor starting from the higher order
 * bits. That is, instead of incrementing the cursor normally, the bits
 * of the cursor are reversed, then the cursor is incremented, and finally
 * the bits are reversed again.
 *
 * This strategy is needed because the hash table may be resized between
 * iteration calls.
 *
 * dict.c hash tables are always power of two in size, and they
 * use chaining, so the position of an element in a given table is given
 * by computing the bitwise AND between Hash(key) and SIZE-1
 * (where SIZE-1 is always the mask that is equivalent to taking the rest
 *  of the division between the Hash of the key and SIZE).
 *
 * For example if the current hash table size is 16, the mask is
 * (in binary) 1111. The position of a key in the hash table will always be
 * the last four bits of the hash output, and so forth.
 *
 * WHAT HAPPENS IF THE TABLE CHANGES IN SIZE?
 *
 * If the hash table grows, elements can go anywhere in one multiple of
 * the old bucket: for example let's say we already iterated with
 * a 4 bit cursor 1100 (the mask is 1111 because hash table size = 16).
 *
 * If the hash table will be resized to 64 elements, then the new mask will
 * be 111111. The new buckets you obtain by substituting in ??1100
 * with either 0 or 1 can be targeted only by keys we already visited
 * when scanning the bucket 1100 in the smaller hash table.
 *
 * By iterating the higher bits first, because of the inverted counter, the
 * cursor does not need to restart if the table size gets bigger. It will
 * continue iterating using cursors without '1100' at the end, and also
 * without any other combination of the final 4 bits already explored.
 *
 * Similarly when the table size shrinks over time, for example going from
 * 16 to 8, if a combination of the lower three bits (the mask for size 8
 * is 111) were already completely explored, it would not be visited again
 * because we are sure we tried, for example, both 0111 and 1111 (all the
 * variations of the higher bit) so we don't need to test it again.
 *
 * WAIT... YOU HAVE *TWO* TABLES DURING REHASHING!
 *
 * Yes, this is true, but we always iterate the smaller table first, then
 * we test all the expansions of the current cursor into the larger
 * table. For example if the current cursor is 101 and we also have a
 * larger table of size 16, we also test (0)101 and (1)101 inside the larger
 * table. This reduces the problem back to having only one table, where
 * the larger one, if it exists, is just an expansion of the smaller one.
 *
 * LIMITATIONS
 *
 * This iterator is completely stateless, and this is a huge advantage,
 * including no additional memory used.
 *
 * The disadvantages resulting from this design are:
 *
 * 1) It is possible we return elements more than once. However this is usually
 *    easy to deal with in the application level.
 * 2) The iterator must return multiple elements per call, as it needs to always
 *    return all the keys chained in a given bucket, and all the expansions, so
 *    we are sure we don't miss keys moving during rehashing.
 * 3) The reverse cursor is somewhat hard to understand at first, but this
 *    comment is supposed to help.
 */
unsigned long dictScan ( dict *d,
                         unsigned long v,
                         dictScanFunction *fn,
                         void *privdata )
{
    dictht *t0, *t1;
    const dictEntry *de;
    unsigned long m0, m1;

    if ( dictSize (d) == 0 ) return 0;

    if ( ! dictIsRehashing (d) )
    {
        t0 = & ( d->ht[0] );
        m0 = t0->sizemask;

        /* Emit entries at cursor */
        de = t0->table[v & m0];
        while ( de )
        {
            fn (privdata, de);
            de = de->next;
        }

    }
    else
    {
        t0 = & d->ht[0];
        t1 = & d->ht[1];

        /* Make sure t0 is the smaller and t1 is the bigger table */
        if ( t0->size > t1->size )
        {
            t0 = & d->ht[1];
            t1 = & d->ht[0];
        }

        m0 = t0->sizemask;
        m1 = t1->sizemask;

        /* Emit entries at cursor */
        de = t0->table[v & m0];
        while ( de )
        {
            fn (privdata, de);
            de = de->next;
        }

        /* Iterate over indices in larger table that are the expansion
         * of the index pointed to by the cursor in the smaller table */
        do
        {
            /* Emit entries at cursor */
            de = t1->table[v & m1];
            while ( de )
            {
                fn (privdata, de);
                de = de->next;
            }

            /* Increment bits not covered by the smaller mask */
            v = ( ( ( v | m0 ) + 1 ) & ~ m0 ) | ( v & m0 );

            /* Continue while bits covered by mask difference is non-zero */
        }
        while ( v & ( m0 ^ m1 ) );
    }

    /* Set unmasked bits so incrementing the reversed cursor
     * operates on the masked bits of the smaller table */
    v |= ~ m0;

    /* Increment the reverse cursor */
    v = rev (v);
    v ++;
    v = rev (v);

    return v;
}

/* ------------------------- private functions ------------------------------ */

/* Expand the hash table if needed */
static int _dictExpandIfNeeded ( dict *d )
{
    /* Incremental rehashing already in progress. Return. */
    if ( dictIsRehashing (d) ) return DICT_OK;

    /* If the hash table is empty expand it to the initial size. */
    if ( d->ht[0].size == 0 ) return dictExpand (d, DICT_HT_INITIAL_SIZE);

    /* If we reached the 1:1 ratio, and we are allowed to resize the hash
     * table (global setting) or we should avoid it but the ratio between
     * elements/buckets is over the "safe" threshold, we resize doubling
     * the number of buckets. */
    if ( d->ht[0].used >= d->ht[0].size &&
         ( dict_can_resize ||
           d->ht[0].used / d->ht[0].size > dict_force_resize_ratio ) )
    {
        return dictExpand (d, d->ht[0].used * 2);
    }
    return DICT_OK;
}

/* Our hash table capability is a power of two */
static unsigned long _dictNextPower ( unsigned long size )
{
    unsigned long i = DICT_HT_INITIAL_SIZE;

    if ( size >= LONG_MAX ) return LONG_MAX;
    while ( 1 )
    {
        if ( i >= size )
            return i;
        i *= 2;
    }
}

/* Returns the index of a free slot that can be populated with
 * a hash entry for the given 'key'.
 * If the key already exists, -1 is returned.
 *
 * Note that if we are in the process of rehashing the hash table, the
 * index is always returned in the context of the second (new) hash table. */
static int _dictKeyIndex ( dict *d, const void *key )
{
    unsigned int h, idx, table;
    dictEntry *he;

    /* Expand the hash table if needed */
    if ( _dictExpandIfNeeded (d) == DICT_ERR )
        return - 1;
    /* Compute the key hash value */
    h = dictHashKey (d, key);
    for ( table = 0; table <= 1; table ++ )
    {
        idx = h & d->ht[table].sizemask;
        /* Search if this slot does not already contain the given key */
        he = d->ht[table].table[idx];
        while ( he )
        {
            if ( key == he->key || dictCompareKeys (d, key, he->key) )
                return - 1;
            he = he->next;
        }
        if ( ! dictIsRehashing (d) ) break;
    }
    return idx;
}

void dictEmpty ( dict *d, void(callback ) ( void* ) )
{
    _dictClear (d, &d->ht[0], callback);
    _dictClear (d, &d->ht[1], callback);
    d->rehashidx = - 1;
    d->iterators = 0;
}

void dictEnableResize ( void )
{
    dict_can_resize = 1;
}

void dictDisableResize ( void )
{
    dict_can_resize = 0;
}

/* ------------------------------- Debugging ---------------------------------*/

#define DICT_STATS_VECTLEN 50

size_t _dictGetStatsHt ( char *buf, size_t bufsize, dictht *ht, int tableid )
{
    unsigned long i, slots = 0, chainlen, maxchainlen = 0;
    unsigned long totchainlen = 0;
    unsigned long clvector[DICT_STATS_VECTLEN];
    size_t l = 0;

    if ( ht->used == 0 )
    {
        return snprintf (buf, bufsize,
                         "No stats available for empty dictionaries\n");
    }

    /* Compute stats. */
    for ( i = 0; i < DICT_STATS_VECTLEN; i ++ ) clvector[i] = 0;
    for ( i = 0; i < ht->size; i ++ )
    {
        dictEntry *he;

        if ( ht->table[i] == NULL )
        {
            clvector[0] ++;
            continue;
        }
        slots ++;
        /* For each hash entry on this slot... */
        chainlen = 0;
        he = ht->table[i];
        while ( he )
        {
            chainlen ++;
            he = he->next;
        }
        clvector[( chainlen < DICT_STATS_VECTLEN ) ? chainlen : ( DICT_STATS_VECTLEN - 1 )] ++;
        if ( chainlen > maxchainlen ) maxchainlen = chainlen;
        totchainlen += chainlen;
    }

    /* Generate human readable stats. */
    l += snprintf (buf + l, bufsize - l,
                   "Hash table %d stats (%s):\n"
                   " table size: %ld\n"
                   " number of elements: %ld\n"
                   " different slots: %ld\n"
                   " max chain length: %ld\n"
                   " avg chain length (counted): %.02f\n"
                   " avg chain length (computed): %.02f\n"
                   " Chain length distribution:\n",
                   tableid, ( tableid == 0 ) ? "main hash table" : "rehashing target",
                   ht->size, ht->used, slots, maxchainlen,
                   ( float ) totchainlen / slots, ( float ) ht->used / slots);

    for ( i = 0; i < DICT_STATS_VECTLEN - 1; i ++ )
    {
        if ( clvector[i] == 0 ) continue;
        if ( l >= bufsize ) break;
        l += snprintf (buf + l, bufsize - l,
                       "   %s%ld: %ld (%.02f%%)\n",
                       ( i == DICT_STATS_VECTLEN - 1 ) ? ">= " : "",
                       i, clvector[i], ( ( float ) clvector[i] / ht->size )*100);
    }

    /* Unlike snprintf(), teturn the number of characters actually written. */
    if ( bufsize ) buf[bufsize - 1] = '\0';
    return strlen (buf);
}

void dictGetStats ( char *buf, size_t bufsize, dict *d )
{
    size_t l;
    char *orig_buf = buf;
    size_t orig_bufsize = bufsize;

    l = _dictGetStatsHt (buf, bufsize, &d->ht[0], 0);
    buf += l;
    bufsize -= l;
    if ( dictIsRehashing (d) && bufsize > 0 )
    {
        _dictGetStatsHt (buf, bufsize, &d->ht[1], 1);
    }
    /* Make sure there is a NULL term at the end. */
    if ( orig_bufsize ) orig_buf[orig_bufsize - 1] = '\0';
}
